1. Field of the Invention
The present invention relates to a diagnostic device of the type having a modality for generating raw data representing contents of a volume, a computer for calculating three-dimensional (3D) medical images from the raw data, an image system, an input device for setting a transfer function required for an algorithm, and a display device for the medical images.
2. Description of the Prior Art
The volume rendering algorithm is gaining increasing importance in the visualization of three-dimensional (3D) volumetric data records in medical technology in the case of computed tomography (CT) and magnetic resonance tomography (MR) and angiography examinations such as CTA or MRA.
The technique of volume rendering is used to reproduce the anatomical spatial relationship between various organs, in order to enhance insight into hidden structures, in particular of blood vessels. Different objects of the same volume such as, for example, blood vessels, bones, skin and soft parts, can be inspected simultaneously. This differentiation is calculated on the basis of a selected object threshold and characterizes the transparency, shading or color.
In the case of a non-pre-segmented volumetric data record, however, this very powerful algorithm requires the definition of a transfer function which allocates each gray-scale value an RGBA value which characterizes a transparency (A) and a color consisting of a mixture of red (R), green (G) and blue (B). The task of the transfer function is to locate and delimit anatomically coherent gray-scale value ranges. It is difficult in this process to locate the boundaries of the gray-scale value ranges, since this process is accomplished by purely empirical approaches such as are described in G. Kindlmann et al. “Semi-Automatic Generation of Transfer Functions for Direct Volume Rendering” in Proceedings Symposium on Volume Visualization '98, pages 79–86, 1998, or S. Fang et al. “Image-Based Transfer Function Design for Data Exploration in Volume Visualization” in Proceedings Symposium on Volume Visualization '98, pages 319–326, 1998. Thus, the majority of current graphical user interfaces offer only the option of defining the transfer function by means of freehand function curves (see FIG. 2) or with the aid of block or trapezoidal functions (see FIG. 3 herein) (compare also R. A. Drebin et al. “Volume Rendering” in Computer Graphics 24(4), pages 65–75, 1988). The basis for this is mostly the gray-scale value histogram or specific prior knowledge of gray-scale value ranges (for example Houndsfield units in the case of CT). Such setting options of the transfer function are complicated and inaccurate, and so it is possible to find the correct transfer function only with difficulty and a disproportionately large time outlay. This means that volume rendering is accepted only with difficulty in the clinical routine among medical practitioners.
German OS 199 55 690 discloses a three-dimensional image system which has a display with segmented elements and a view selector which permits identification of special features. The selection of the volumetric elements is made on the basis of a comparison with defined threshold values corresponding to the opacity.